In the broadband CDMA (Code Division Multiple Access) system, which is expected to become a standard system for next-generation cellular phones, signals are spread over wide spectrum by using a spreading code, and thus enabling one frequency band to be shared by plural channels. In addition, there is another advantage of high secrecy. On the other hand, in the CDMA system, it is necessary to provide a receiver with a circuit for eliminating the spreading code differently from a conventional narrow-band modulation system, which enlarges the circuit size.
In a CDMA transmitter, transmission data D(t) are multiplied by a spreading code c(t) to be a spectrum-spread transmitter signal s(t), and thereby transmitted. The transmission data D(t) include a certain data sequence at the first part. The data are called fixed data and denoted by xi(t). The transmitter signal s(t) is sent through a transmission line and received by a CDMA receiver. The signal received at the CDMA receiver is denoted by r(t).
In the following, a description will be given of the case where data are transmitted by using one of plural pieces of fixed data x1(t), x2(t), x3(t), . . . , xm(t) as the above-mentioned fixed data xi(t) (m: an integer 2 or more; 1 (one)≦i≦=m). The CDMA receiver is previously provided with the values of the plural pieces of fixed data x1(t), x2(t), x3(t), . . . , xm(t), however, unable to decide which one of the pieces of fixed data x1(t), x2(t), x3(t), . . . , xm(t) are to be sent.
FIG. 9(A) illustrates a conventional synchronous acquisition device for a CDMA receiver. The synchronous acquisition device includes first to nth paths (n: an integer 2 or more), which are supplied with first to nth branch signals obtained by branching a receiver signal r(t), respectively.
The synchronous acquisition device further includes first to nth delay units 101, 102, 103, . . . , 10n connected to the first to nth paths. The first to nth delay units 101, 102, 103, . . . , 10n output first to nth delayed signals r1(t), r2(t), r3(t), . . . , rn(t), respectively, by giving different amounts of delays τ1, τ2, τ3, . . . , τn to each of the first to nth branch signals. That is, the jth branch signal that branches into the jth path of the first to nth paths (1 (one)≦j≦n) is delayed by the jth delay unit 10j and outputted as the jth delayed signal rj(t).
Subsequently, in the synchronous acquisition device, first to nth maximum correlation value generation units 151, 152, 153, . . . , 15n connected to the first to nth delay units 101, 102, 103, . . . , 10n generate first to nth maximum correlation values based on the first to nth delayed signals r1(t), r2(t), r3(t), . . . , rn(t).
Then, a synchronism-acquiring maximum value detector 14 connected to the first to nth maximum correlation value generation units 151, 152, 153, . . . , 15n detects a maximum value of the first to nth maximum correlation values to acquire synchronism.
FIG. 9(B) illustrates a detail of the nth maximum correlation value generation unit 15n of the first to nth maximum correlation value generation units 151, 152, 153, . . . , 15n in the synchronous acquisition device shown in FIG. 9(A). The other maximum correlation value generation units have the same structure as that of the nth maximum correlation value generation unit 15n.
As shown in FIG. 9(B), the nth maximum correlation value generation unit 15n branches a signal rn(t) delayed by the delay unit 10n into paths of the same number as m pieces of fixed data (namely, first to mth paths) as first to mth branch signals, and the first to mth branch signals are inputted to first to mth correlation devices, respectively. The first to mth correlation devices multiply the first to mth branch signals by the products of the first to mth pieces of fixed data x1(t), . . . , xm(t) and the spreading code c(t), respectively, and integrate first to mth multiplication results by an integrator′ 12 to output first to mth correlation values y1n(t), . . . , ymn(t). The first to mth correlation values y1n(t), . . . , ymn(t) outputted from the first to mth correlation devices get high when the first to mth branch signals are synchronized with the products of the first to mth pieces of fixed data x1(t), . . . , xm(t) and the spreading code c(t), when not, the values come low. A maximum value detector 14′ corresponding to the path of the nth maximum correlation value generation unit 15n detects a maximum value yin(t) max (1 (one)≦i≦m) of the first to mth correlation values y1n(t), . . . , ymn(t) as the nth maximum correlation value. In more detail, the maximum value detector 14′ corresponding to the path of the nth maximum correlation value generation unit 15n outputs the maximum correlation value yin(t) max and i corresponding to the maximum correlation value yin(t) max.
To sum up, in FIG. 9(A), the jth (1 (one)≦j≦n) maximum correlation value generation unit 15j of the first to nth maximum correlation value generation units 151, . . . , 15n outputs the jth maximum correlation value yij(t) max and i corresponding to the jth maximum correlation value yij(t) max. The synchronism-acquiring maximum value detector 14 in FIG. 9(A) detects a maximum value max {yij(t)max} of the maximum correlation values yij(t) max (1 (one)≦j≦n) to acquire synchronism, and outputs the detected maximum value max {yij(t)max}, and i and j corresponding to the maximum value max {yij(t)max}.
As is described above, inside the maximum correlation value generation units 151, 152, 153, . . . , 15n of FIG. 9(A), m integrators 12′ are lined up in parallel, which makes circuit size very big.
Incidentally, as shown in FIG. 9(B), each of the integrators 12′ includes an adder 16 for receiving an input signal at its first input terminal, and a delay element 17 composed of a latch L for delaying an output signal from the adder 16 by 1-symbol-time and outputting the delayed signal as an integrator output signal to input it to the second input terminal of the adder 16.
In the following, the operation of the synchronous acquisition device will be explained with reference to FIGS. 9(A) and 9(B).
Transmission data D(t) are expressed as follows:D(t)=xi(t) (0≦t<t0)D(t)=d(t) (t0≦t)  (1)In which d(t) denotes information data. That is, first (0≦t<t0), the transmission data includes fixed data xi(t) (1 (one)≦i<m), and subsequently (t0≦t), includes the information data d(t). The information data d(t) need to be synchronized with the fixed data xi(t) when received.
A transmitter signal s(t) is obtained by multiplying the transmission data D(t) by a spreading code c(t). Assuming that 0≦t<t0 with regard to time t, the transmitter signal s(t) is expressed as:s(t)=xi(t)c(t) (0≦t<t0)  (2).
An output yij(t) of the correlation device can be obtained by multiplying the receiver signal r(t)=s(t) by the spreading code c(t) and the fixed data xi(t) after giving a delay τj(1 (one)≦j≦n) in each path, and integrating the product by N-symbol-time as following expression. Then, a maximum value is detected among the paths to acquire synchronism.yij(t)=∫0t0r(t−τj)xi(t)c(t)dt  (3)